Photoelectric electron multiplier



Patented June 11, 1946 PHOTOELECTRIC ELECTRON MULTIPLIER Robert B.Janes, Verona, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application October 8, 1940, Serial No. 360,2559 Claims. (01. 250-175) My invention relates to photoelectric electronmultiplier tubes and the method of manufacturing such tubes, andparticularly to such tubes containing within a single envelope aphotoelectric cathode and one or more secondary electron emittingelectrodes.

Metals such as antimony, arsenic and bismuth have been photosensltizedby evaporating one of these metals and condensing the metal from thevapor stage upon a foundation, whereupon, prior difllcult to separatethe processes of photosensitization from the processes of secondaryemissive sensitization because the electrodes being in a common envelopeare subjected to the same atmosphere and to substantially the sametreatment irrespective of-the desired photo-emissive andsecondary-emissive properties.

It is an object of my invention to provide a method of manufacturingphotosensitive secondary electron multiplying tubes wherein both thephotocathode and the secondary electron emitting electrodes areconstructed and treated by a common process so as to obtain highsensitization. It is a further object of my invention to provide a tubeof the type described having high photosensitivity and high secondaryelectron emissivity and in which these objects may be obtained withsimilar treatment; and it is a still further object to provide a tubewherein a single process of sensitization produces optimumphotosensitivity of the cathode and optimum secondary electronemissivity of the secondary electron emitters. These and other objects,features, and advantages of my invention will be apparent when taken inconnection with the following description of my invention and theaccompanying drawing, in which,

Figure 1 shows a type of tube to which my inquent treatment with analkali metal such as caesium produces optimum photosensitivity of thephotocathode and optimum secondary electron emissivity of the secondaryelectron emitting electrodes. More particularly, and in accordance withmy invention, I provide a coating of antimonsuarsenic, or bismuth of apredetermined thickness on the photocathode foundation and a coating ofthe same metal from this group but of a diiferent thickness than thepredetermined thickness on the secondary electron emitting electrodefoundations so that when sensitized by a common sensitizing treatment,optimum photosensitivity and optimum secondary electron emissivity willresult. While I will describe my invention with respect to the use ofantimony and caesium in a particular type of tube, it is to beunderstood that I am not limited to the particu-' lar type of structuredescribed, my invention broadly relating to all tubes havingphotocathodes and secondary electron emitting electrodes and to suchtubes having electrodes comprising antimony, arsenic, or bismuthsensitized with an alkali metal.

Referring to the drawing, I provide a tube shown in Figures 1 and 2having a single envelope or bulb l enclosing both the photocathode andsecondary electron emitting electrodes wherein the cathode 2 issupported to intercept light projected through thewall of the envelopeand in a relationship with a secondary electron emitter 3 for thepurpose of withdrawing the electron emission from the photocathode 2. Todirect the electrons from the photocathode 2 to the secondary electronemitter 3 I provide a directing electrode which may be in the form of acoarsely wound grid 4 which is electrically connected to the cathode 2so that electrons emitted therefrom are directed toward the secondaryemitter 3. A further secondary electron emitter 5 is provided oppositethe emitter 3 and it is well known in the art that an additionalplurality of emitters 6 and I may be provided for further secondaryelectron amplification of the photo-emission from the photocathode 2. Ananode 8 which may likewise be in the form of a wire mesh grid surroundedby an anode shield 9 is provided to collect the electron flow followingthe electron multiplication. Myinvention does not relate to the specificarrangement or structural details of the secondary electron emitters,but it has been found that the emitters 3 may be structurally similarone to the other and to the emitter 3, and likewise the emitters istructurally similar to each other and to the emitter 5.,

In accordance with my invention the cathode 2 comprises a foundation itwhich may be of nickel or other base metal carrying on its surface athin film ll of antimony, bismuth, or arsenic,

the film being of predetermined thickness. This film will be referred tohereinafter as an antimony film and may be applied to a conductingfounda tion as described in my above copending application. Thesecondary electron emitter 3 is likewise constructed of a foundation I2of metal such as nickel and likewise carries on its surface, which is tobe secondary electron-emissive, a thin film l3 of antimony, but of athickness greater than the thickness of the film II on the cathode 2.The secondary electron emitters 5, 6, and 'I are likewise provided withthis thin film of antimony having a thickness greater than the thicknessof the antimony film II on the photocathode 2.

The foundation for the various emitting electrodes may be provided withthe film of antimony prior to their assembly in the evacuated envelopeor bulb l by sealing the various electrode foundations in a separateenvelope or bell jar provided with a. source of antimony which may beheated, vaporized and condensed on the various foundations. I have foundthat the thickness of the antimony film on photocathode 2 is preferablyabout 500 angstroms (A.), Whereas I have found the thickness of theantimony film on the secondary emitter foundations to be preferablytwice as great, or approximately 1000 A. The thickness of the antimonyfilms may be determined by the amount of antimony evaporated. Thus, toobtain an antimony film of 1000 A. thickness, I have evaporated 160milligrams of antimony supported on a tungsten filament along an axissurrounded by a group of the electrode foundations to be coated and at adistance of 3 from the antimony-bearing filament. Similarly, theevaporation of 80 milligrams of antimony will produce a film of 500 A.thickness when surrounded by one or more cathode foundations at asimilar distance. It is apparent from Figure 2 that the cathode I isrelatively fiat, whereas the emitters 3 and 6 are arcuate-shaped and theevaporation of the antimony or other metal thereon may be done prior tothe shaping of the electrode foundations although for electrodes havingconsiderable area it is desirable to pre-form the foundations so thatthey are arcuate-shaped with their centers of curvature at the sourcefrom which the antimony is evapo- .rated.

Following the condensation of antimony to a different thickness on thephotocathode foundation with respect to the thickness on the secondaryemitter foundations, I support the electrodes within the envelope lbetween two supports 20 and 2| which may be of mica perforated toaccurately position the electrodes. Following the assembly, the envelopeI is evacuated to a high degree of vacuum and the entire assembly andtube baked at a temperature of 150 C. for a period of from 4 to 10minutes although I have found that this baking step may be omitted withsubstantially no injurious effects. Following the baking step, if notomitted, and prior to any oxidation whatsoever of the antimony films, Ivaporize alkali metal within the envelope such as by heating a tab 22containing an alkali metal or compound in the form of a pellet 23, suchas by high frequency induced currents. In accordance with my priorteaching and throughout the vaporization of the alkali metal I maintainthe temperature of the antimony coated electrodes at such a temperatureas to cause an alloying of 4 the alkali metal with the antimony films IIand I3. The temperature to produce such alloying of the alkali metalwith the antimony may vary over wide limits extending from roomtemperature up to 200 C. and since all of the electrodes are within asingle envelope they are maintained at substantially the sametemperature. The evidence that an alloy of antimony and caesium isformed resides in the fact that the antimony film prior to thevaporization or introduction of caesium in vapor form within theenvelope has substantially. no photosensitivity and the secondaryelectron emissivity of the secondary emitters is very low. Immediatelyupon the liberation of caesium, the photosensitivity of the cathode andthe electron emissivity of the secondary emitters increases even whenthe bulb I is maintained at room temperature. The phenomenon of thecathode antimony film becoming photosensitivite and the antimony on thesecondary emitters becoming highly secondary-electron emissive isbelieved to be due to the formation of the antimonycaesium alloy, andsince the caesium condenses on the antimony film from the vapor stage inwhich state it is quite hot, this formation of an alloy apparentlyoccurs even at room temperature. The caesium is preferably slowlyevaporated and for this reason the pellet 23 may be located outside ofthe envelope in a separate tubulation for better control of the amountof caesium introduced within the bulb I. Somewhat more uniformphotosensitivity over the entire area of the cathode may be obtained bythis latter method makin possible a first condensation of caesium vaporon the wall of the tube followed which the tube is baked in an oven to atempera- V ture of to C. until the photosensitivity has reached a newand higher maximum. I have found some tubes to reach this second maximumfollowing a baking of a few minutes, whereas other tubes require bakingfor several hours. Continued baking after a maximum sensitivity has beenobtained does not harm the tube so that after sealing off the bulb Ifrom the exhaust system a large quantity of tubes may be baked togetherin a large oven for a time sufficient for all of the tubes to attainmaximum photosensitivity.

It may be noted that the above process is based on maximumphotosensitivity as a reference. It is also possible and sometimespreferableto collect not only the photo-emission from the cathode 2 suchas by maintaining the secondary emitters at a common potential, but toapply progressively more positive potentials to the secondary emitters,whereupon the amplified photo-emission is collected by the anode 8. Thislatter method may in some cases produce a better overall response,although tubes made in accordance with my invention produce optimumphotoemission and optimum secondary electron emission because of thepredetermined thickness of antimony films used on the electrodes.

Ihave found for optimum photosensitivity and metal to antimony explainsthe greatly improved results of a tube processed in accordance with myinvention. This improvement is due to the fact that the quantity ofcaesium per unit alloy volume or density of caesium in the antimonycaseium alloy of the photocathode is greater than the density of caesiumin the antimony alloy of the secondary emitter. With antimony films ofdifferent thickness the same amount of caesium may be used to obtaindifferent caesium densities. I have found that in the range of antimonyfilm thicknesses used and for optimum photosensitivity and secondaryelectron emissivity the amount of caesium absorbed by the antimony filmsis substantially independent of the film thickness. Thus per unit areathe cathode film which preferably is about one-half the thickness of thesecondary emitter film absorbs approximately the same amount of caesiumas the secondary emitter film. The relative thickness of the filmsremains substantially the same but the caesium density of the cathodefilm is approximately twice the caesium density of the emitter film.Furthermore, while I have found for best results that the thickness ofthe antimony cathode film is approximately one-half that of the antimonysecondary emitter film, I do not wish to be limited to this particularratio. Depending on the amount of caesium used a condition may bereached at which the ratio of secondary emitter antimony film thicknessto cathode film thickness may be as high as 3 to 1, or as low as 1.5to 1. Furthermore, for greater or smaller amounts of caesium introducedinto the envelope containing the electrodes, the thickness of the filmsmay be correspondingly increased or decreased.

While I have described my invention in connection with the use ofantimony, it is to be understood that I do not wish to be limited tothis particular metal, since I have found arsenic and bismuth to besatisfactory as an equivalent of antimony. Therefore, while I haveindicated the preferred embodiments of my invention of which I am nowaware and have also indicated only certain specific applications forwhich my invention may be employed, it will be apparent that myinvention is by no means limited to the exact forms illustrated or theuse indicated, but that many variations may be made in the particularstructure used and the purpose for which it is employed withoutdeparting from the scope of my invention as set forth in the appendedclaims.

I claim:

1. An electron discharge device of the electron multiplying typeincluding a cathode of a metal alloyed with an alkali metal and asecondary electron emitter of a metal alloyed with an alkali metal, theratio of alkali metal to total alloy volume of said cathode beinggreater than the ratio of alkali metal to total alloy volume of saidemitter.

2. An electron discharge device of the photocathode-secondary electronmultiplier type comprising a photocathode of a metal selected from thegroup of metals consisting of antimony, arsenic, and bismuth alloyedwith an alkali metal, and a secondary electron emitting electrodeincluding an alloy of the same metals as said cathode, the density ofthe alkali metal in the alloy of said photocathode being greater thanthe density of alkali metal in the alloy of said secondary electronemitting electrode.

3. An electron discharge device including an antimony-alkali metal alloyphotocathode and an antimony-alkali metal alloy secondary electron 6emitter the amount of alkali metal per unit volume of said cathode alloybeing greater than that of said emitter alloy.

4. A photoelectric electron multiplier comprising a single evacuatedenvelope, a cathode comprising an antimony caesium combination, asecondary electron emitter comprising an antimony caesium combinationthe ratio of caesium to antimony comprising said cathode being greaterthan the ratio of caesium to antimony comprising said secondary electronemitter.

5. A photoelectric electron multiplier comprising a cathode foundationand a secondary electron emitter foundation, a coating on said cathodefoundation comprising a mixture of antimony and caesium, and a coatingof antimony and caesium on said emitter foundation, the thickness of theantimony on said foundations differing in a ratio of substantially twoto one.

6. The method of manufacturing a photoelectric device including aDhotoelectrically-sensitive cathode and a secondary electron emitterwhich comprises depositing a metal selected from the group of metalsconsisting of antimony, arsenic. and bismuth on a supporting foundationto a predetermined thickness depositing a similar metal from said groupof metals on a second foundation to a. thickness greater than saidpredetermined thickness and subjecting said foundations to alkali metalvapor to form alkali metal alloys having diiferent densities of alkalimetal.

'7. The method of manufacturing a photoelectric device including aphotocathode and a secondary electron emitter which comprises depositingto a predetermined thickness on a. supporting foundation a metalselected from the group of metals consisting of antimony, arsenic, andhismuth, depositing a similar metal from said group of metals on asecond foundation to a thickness substantially twice said predeterminedthickness, sealing said foundations in a single envelope, evacuating theenvelope, and subjecting said foundations to alkali metal vapor to formalkali metal alloys wherein the ratio of alkali metal to the metalselected from said group of metals is greater on said first-mentionedfoundation than on said second foundation.

8. The method of manufacturing a photoelectric secondary electronmultiplier which comprises forming a pair of electrode foundationshaving films of diiferentthickness of metals selected from the group ofmetals consisting of antimony, arsenic, and bismuth, and subjecting saidfilms to alkali metal vapor to formalkali metal alloys having differentalkali metal densities.

9. The method of manufacturing an electron discharge device including acathode and a secondary electron emitter which comprises forming acoating of antimony on a cathode foundation, forming a coating ofantimony on a secondary emitter foundation to a thickness between oneand one-half to three times the thickness of antimony on said cathodefoundation, sealing each of said coated foundations within a singleenvelope, evacuating said envelope and simultaneously subjecting saidcoated foundations to alkali metal vapor prior to any step of oxidationthereof within said envelope to form an, antimony alkali metalcombination wherein the ratio of alkali metal to antimony comprisingsaid cathode is reater than the ratio of alkali metal to antimonycomprising said secondary emitter.

ROBERT B. JANEB.

